This invention relates to an optical spectrometer device. The device is generally usable in the wide range of measurements where spectrometers are employed. Specific technical features make this instrument well suited to field measurements.
An inventory of earth resources can be effectively accomplished by spectrometer observations from an airplane or satellite. Information is obtainable since various substances, life forms and structures reflect and radiate energy at various wavelengths in characteristic fashions. Agronomists, botanists, geologists, hydrologists, and many others can extract valuable information from the images made from narrow wavelength bands.
In order to provide useful spectrally dispersed images of the earth's surface, an imaging spectrometer is flown aboard a high altitude platform such as a space shuttle, aircraft, or free-flying satellite. Information gathered by the imaging spectrometer can be stored internally for later retrieval or transmitted to an earth or satellite data receiving station.
Spectrometers function using the principle of dispersion of light which occurs as rays of light are deviated, typically by a diffraction grating or refracted through a prism. Diffraction gratings behave optically like a multiplicity of very narrow individual slits which cause light rays to be deviates in accordance with the wavelength of those rays. Prisms cause dispersion of light since the angle of deviation of a light ray as it passes through a prism is a function of its wavelength due to the fact that optical materials exhibit differing indexes of refraction dependent upon the wavelength of light passing therethrough. Spectrometer systems using prism dispersing elements have inherent advantages over those using diffraction grating-type dispersion elements since they are more efficient in terms of light transmission and less troubled by stray light. Accordingly, many current designs of imaging spectrometers employ prism-type dispersion elements. Imaging spectrometers intended for field use employ a fore-optics section which images a portion of the earth's surface onto a slit to define the field-of-view. Light transmitted through (or reflected from) the slit is then dispersed using a diffraction grating or prism. The dispersed light is imaged on a focal plane, which would typically be comprised of an array of minute photosensitive elements. As the spectrometer platform traverses the earth's surface, a swath of the earth's surface is imaged. The information gathered by the spectrometer can be later processed to produce images of the areas studied which indicate the emission and/or reflection of various wavelengths of light from various points on the earth's surface.
A number of designs for imaging spectrometer devices are currently known. These devices typically have a fore-optics section which produces an image of a selected portion of the earth's surface, a slit, and a spectrometer portion which disperses the slit image. These devices, however, tend to be bulky and are essentially single-purpose instruments, i.e., imaging spectrometers. Further, these systems typically require a non-planar focal plane assembly which significantly complicates fabrication of the focal plane with its array of detectors. Moreover, currently envisioned imaging spectrometers require that the fore-optic systems include a collimator to make all the light rays parallel. Collimation is necessary to control aberrations which result when non-collimated light is transmitted through the prism dispersing element. Current imaging spectrometers further are relatively bulky instruments since the optical ray path lengths tend to be fairly long.